Field of the Invention
The present invention relates to an illumination optical system that introduces a light flux from a light source to an illumination surface, and particularly to an illumination optical system suitable for an image projection apparatus in which a light modulation element such as a digital micromirror device (hereinafter referred to as a DMD) is disposed at the illumination surface.
Description of the Related Art
The image projection apparatus mentioned above introduces the light (illumination light) from the light source to the light modulation element disposed at the illumination surface and projects the light (image light) modulated by the light modulation element onto a projection surface through a projection optical system to display a projected image.
The DMD is a typical light modulation element and is generally configured to have an entirely rectangular modulation area as shown in FIG. 8A. The modulation area includes multiple square micromirrors arranged in a matrix. Each of the micromirrors is rotated about a diagonal line “d” as an axis between two rotational positions, which are an ON position and an OFF position, according to a pixel value of each pixel of image information. For example, as shown in FIG. 8B, the ON position is a position at which the micromirror is tilted by +12 degrees with respect to a modulation surface (horizontal plane in the figure) that is a planar surface on which the multiple micromirrors are arranged in the DMD, and the OFF position is a position at which the micromirror is tilted by −12 degrees with respect to the modulation surface.
The illumination optical system causes the illumination light to reach each micromirror from a direction tilted by 24 degrees with respect to a normal to the modulation surface. The micromirror at the ON position reflects the reaching illumination light in a direction (normal direction) in which the normal to the modulation surface extends. Since the projection optical system is disposed in the normal direction, the reflected light is projected onto the projection surface.
On the other hand, the micromirror at the OFF position reflects the reaching illumination light in a direction different from the normal direction to the modulation surface (that is, in a direction not traveling to the projection optical system), so that the reflected light is not projected onto the projection surface. Rotating the micromirror at high speed between the ON and OFF positions during one flame of the image information and changing a ratio of a period of time during which the micromirror is located at the ON position to a period of time during which the micromirror is located at the OFF position enables performing tone expression to display images having various tones.
The illumination optical system shapes a divergent light flux from the light source such that a cross-sectional shape of the light flux (hereinafter referred to as “a light flux cross-sectional shape”) becomes close to a rectangular shape of an effective area of the DMD to enhance utilization efficiency of the light. On the illumination surface at which the DMD is disposed, an area where the light reaches (hereinafter referred to as “an illumination area”) is set, in consideration of manufacturing error and position accuracy of optical elements constituting the illumination optical system, so as to have a margin around the modulation area of the DMD.
However, the illumination light introduced to the DMD from the above-described tilted (oblique) direction with respect to the normal to the modulation surface causes distortion and defocus, as shown in an upper part of FIG. 9, in an azimuth corresponding to where an incidence azimuth of the illumination light to the DMD (that is, a rotation direction of the micromirror) is projected to the illumination surface. Specifically, when seen from the normal direction to the modulation surface, the illumination light reaches the DMD from an azimuth of +45 degrees with a tilt of 24 degrees with respect to the normal to the modulation surface. With such an oblique illumination, on a section (xz section) including the azimuth of +45 degrees, an optical axis of the illumination optical system does not intersect orthogonally with the modulation surface, so that an apparent magnification of the illumination area increases.
In the upper part of FIG. 9, “a” represents a size of the illumination area on the xz section when the illumination light reaches the illumination area from a direction tilted by an angle θ with respect to the normal to the modulation surface of the DMD. In a lower part of the FIG. 9A, “b” represents a size of the illumination area in a yz section when the illumination light reaches the illumination area from a direction not tilted with respect to the normal to the modulation surface, which is defined as 1. With these representations, the size a is expressed by 1/cos θ. The tilt of the normal to the modulation surface of the DMD with respect to the optical axis of the illumination optical system causes a shift of the micromirrors in the modulation surface with respect to a focal position of the illumination optical system, which generates defocus. On the other hand, in a direction of −45 degrees, the optical axis of the illumination optical system is not tilted with respect to the normal to the modulation surface, so that the apparent magnification does not change and the defocus is not generated.
As a result, as shown in FIG. 10, the illumination area on the illumination surface is distorted from its original rectangular shape in its diagonal direction and also has a defocused portion, and thereby the illumination area has a shape like a parallelogram (rhomboid). Such a distorted illumination area is undesirable because when its oblique outline overlaps the modulation area of the DMD, part of the projected image becomes dark.
For this reason, it is necessary to secure a large margin around the illumination area so as to prevent the oblique outline of the illumination area from overlapping the modulation area of the DMD. However, such a large margin decreases the light utilization efficiency and thereby makes the projected image dark. Furthermore, the defocused portion of the illumination area has a lower illuminance as compared with a non-defocused portion, which makes part of the projected image corresponding to the defocused portion dark and thereby decreases a brightness evenness of the projected image.
Japanese Patent Laid-Open No. 2000-267044 discloses a method of correcting a rhomboidal distortion of an illumination area on an illumination surface (DMD) by forming an exit surface of an illumination optical system (rod integrator) into a rhomboidal shape. Japanese Patent Laid-Open No. 2004-45718 discloses a method of correcting a rhomboidal distortion of an illumination area by rotating and decentering at least one optical element (lens) constituting part of an illumination optical system with respect to an optical axis of the illumination optical system.
However, the method disclosed in Japanese Patent Laid-Open No. 2000-267044 causes an illumination light to obliquely reach the DMD and therefore a modulation surface of the DMD is not parallel to the exit surface of the rod integrator. As a result, defocus is still generated in a diagonal direction of a rectangular illumination area.
On the other hand, in the method disclosed in Japanese Patent Laid-Open No. 2004-45718 the rotation and decentering of the lens causes eccentric aberration, which decreases an imaging performance of the illumination optical system for the DMD. As a result, a sharpness of the illumination area decreases and thereby illuminance thereof decreases as in the defocused portion.